Michigan Governor Rick Snyder has signed a right-to-work bill limiting union power into law in his state, he announced in a press conference on Tuesday evening.

"I view this as an opportunity to stand up for Michigan's workers—to be pro-worker," Snyder said.

"I don't view this as anti-union at all...I believe this is pro-worker."

Earlier on Tuesday the GOP controlled state house approved a law that would make the payment of union dues voluntary for private-sector unions and most public-sector unions (police and firefighters would be exempt.) The bill was approved by a vote of 58-51.

In anticipation of the vote, thousands of protesters descended on the statehouse as early as 5am on Tuesday. Later in the day, the demonstrations moved to the Romney building- which is named after former Michigan Gov. George Romney (father of Mitt)- where Snyder has an office.

There's symbolism in the location. During his tenure as governor, Romney signed the first bills in the state that gave collective bargaining rights to public-sector employees.

The protests continued on through the day on Tuesday, and by the afternoon riot police surrounded the Romney building in an effort to keep protesters out.

Michigan state police told the Associated Press that pepper spray was used to calm a protester, but there was no arrest in that incident. Two separate arrests occurred on Tuesday when two individuals tried to get into the Romney building.

Michigan now becomes the 24th state to pass right-to-work legislation, but the passage of a such a law in union-heavy Michigan is particularly divisive in the state. Michigan is the birthplace of the powerful United Auto Workers organizatiom and union representation in Michigan is among the highest in the nation; roughly 17.5 percent of the state's labor force is unionized.

Although Snyder has signed the bill into law, it will still be possible to put it on the ballot in 2014, when Snyder is expected to run for re-election. The bill includes appropriations, which means that it will automatically become law if signed, but the state constitution allows for voters to invoke a referendum to "approve or reject" the law.

Opponents of the law will have 90 days after the legislature adjourns to gather 8 percent of the total votes cast in the last gubernatorial race, which were more than 3 million.

If they succeed, the law will be placed on the ballot and subject to a statewide vote.